Powder coating compositions are extremely desirable for use in painting substrates in that they are essentially free of organic solvents which are conventionally utilized in liquid paint systems. Accordingly, economic and social benefits are realized through the use of powder coatings by achieving reduced air pollution, reduced energy use and, not insignificantly, reduced health and fire hazards.
Moreover, the use of powder coatings, as opposed to liquid paint formulations, also offers the advantages of, among other things, generally able to effectively coat a substrate in just one application; have high material yield (typically up to about 99% efficiency); and superior corrosion resistance.
Generally, powder coating compositions are prepared by first forming the desired resin material, usually by solution polymerization wherein the monomeric constitutients are dissolved in an organic solvent. After polymerization and isolation of the resulting resin, it is mixed with other resin constituents, if desirable; crosslinking agents; pigment, if desirable; and other conventional additives, such as, plasticizers, and the like. These components are then typically intimately mixed by melting the mixture ("melt mixing"), extruding the melted mixture and then cooling and crushing. This is then generally followed by a sequence of operations involving grinding, sifting, separation, and filtering, followed by more sieving.
To provide a desired color match, a number of different powder coatings, each possessing its own respective color, generally must be dry mixed. Typically, however, such dry mixing does not readily provide a true intimate mixing of the various colored constitutents. As a result, it is not unusual to see, upon visual inspection of a substrate coated with such a dry mixed composition, areas of the individual colors and a general non uniformity of the final color.
The substantially non-spherical, irregularly shaped particles formed as a result of the crushing and grinding operations noted above may have an undesirable effect on the uniformity and continuity of the resulting coating formed on the substrate after being heat cured. This may be due to a number of different phenomena occurring as a result of these non-spherical, irregularly shaped particles. For example, as a result of such irregularly shaped particles, agglomerates are easily and typically formed. This generally produces a non-uniform, uneven layer of the coating composition on the substrate, even before it is subjected to a heat curing step. So too, during the heat curing step itself, the powder coating composition is hampered by these irregularly shaped particles to effectively flow out over the substrate in a uniform and even manner. Still further, these irregularly shaped particles may also affect the performance of electrostatically spraying the powder coating composition onto a substrate, the typical means by which such powder coating compositions are applied. Thus, such irregularly shaped particles may cause a non-uniform charging of the particles which results in undesirable clumping and corresponding uneveness in the coating layer on the substrate. So too, the irregularly shaped particles may not be able to be charged as well as the more spherical particles of the present invention.
Among other things, a need clearly exists to provide coreactable powdered coating compositions which are not only stable when stored until ready for use, but which can provide a true intimately mixed color blend which is uniform in color even when examined under close visual inspection. Moreover, in conjunction with these needs, a still further need exists for providing powdered coating compositions having the properties of being able to be applied onto a substrate as a more generally uniform and continuous layer and having improved flow out upon heat curing so as to ultimately produce coated substrates having more uniform and more continuous coating films thereon.